Question

c=1+3+5+....77+79 va rogg repedee

Answer 1

c=1+3+5+....77+79
c = 2×1-1 + 2×2-1 + 2×3-1 +...+ 2×39-1 + 2×40-1
c = 2(1+2+3+...+40) -( 1+1+1+...+1)
de 40 de ori
c = 2×41×40:2 - 1×40
c = 2×41×20-40
c = 2×820-40
c = 1640-40
c = 1 600

Answer 2

    
[tex]\displaystyle\\ C=1+3+5+\cdots+77+79\\\\ \text{Calculam numarul de termeni:}\\\\ n = \frac{79-1}{2}+1 = \frac{78}{2}+1 = 39+1 = 40~\text{termeni}\\\\ C = 1+3+5+\cdots+77+79 \\\\ C= \frac{n(79+1)}{2}= \frac{40\times 80}{2}= 40\times 40 = 40^2 = \boxed{\bf 1600}[/tex]